Mycobacterium tuberculosis infects approximately 2 billion people worldwide and can persist for the lifetime of the host. Loss of immune control and progression to active tuberculosis (TB) disease occurs in a subset of infected individuals, however the precise mechanisms leading to this immunological failure are not known. Persistent antigen stimulation in human and animal models of chronic infections leads to antigen-specific T cell dysfunction, characterized by progressive loss of cytokine production, proliferative capacity, and cytolytic activity. This functional 'exhaustion' of T cells has been previously described in the context of chronic viral infections, and has been associated with antigen-driven upregulation of inhibitory receptors that negatively regulate antigen-specific T cells. Negative regulatory receptors expressed on activated T cells include members of the B7 family: programmed death 1 (PD-1), cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), and B and T lymphocyte attenuator (BTLA), as well as members of the immunoglobulin superfamily: CD160 and T cell immunoglobulin domain and mucin domain 3 (TIM-3). The relationship between chronic mycobacterial antigen stimulation and expression of receptors that negatively regulate activated M. tuberculosis-specific T cells has not been addressed. The underlying hypothesis of this proposal is that expression of negative regulatory receptors is related to mycobacterial antigen levels, and that upregulation of negative regulatory receptors is associated with dysregulation of protective M. tuberculosis- specific T cell responses in persons with pulmonary TB disease. The following specific aims will be addressed: (1) Characterize expression of negative regulatory receptors on cells from the lungs of individuals with latent and active pulmonary TB; (2) Characterize negative regulatory receptors in peripheral blood of individuals with latent and active pulmonary TB, before and after initiation of treatment; (3) Determine the effect of blockade of negative regulatory pathways on M. tuberculosis-specific T cell function. These studies will focus on individuals in Cape Town, South Africa with different mycobacterial antigen levels, including asymptomatic latent TB infection (LTBI), acid-fast bacilli (AFB) sputum smear- negative/culture-positive pulmonary TB, and AFB sputum smear-positive pulmonary TB. Elucidating mechanisms contributing to immunological failure in persons who develop active TB disease is essential for a better understanding of M. tuberculosis immunopathogenesis, and will facilitate new avenues of research on immunotherapeutic interventions and the rational design of novel TB vaccines. PUBLIC HEALTH RELEVANCE: In the majority of individuals infected with Mycobacterium tuberculosis, the immune response successfully contains the bacteria in a latent state and the host remains asymptomatic. Investigation of mechanisms of the cellular immune response that regulate M. tuberculosis- specific T cell responses in persons with latent and active disease is essential for a better understanding of why the immune response fails in some infected individuals who progress to develop active tuberculosis disease; such studies will open up new avenues of research on immunotherapeutic interventions and the design of more effective tuberculosis vaccines.